Pluto used to be the ninth planet in our Solar System. Then in 2006 everything changed. Dr Matt Agnew explains why... and it's a better story than you think.

In this episode of Why Though?, we travel back to 1930 when astronomer Clyde Tombaugh spotted a tiny moving dot in the night sky and made one of the biggest discoveries in space history. We find out what happened when scientists discovered Eris... a Pluto-like world that was even heavier than Pluto. And we learn the three-rule checklist that decides what actually counts as a planet.

Of course Matt's dog Pluto makes a cameo.

What you'll learn:

- Pluto was discovered in 1930 and taught as the ninth planet for over 70 years

- Better telescopes revealed a whole region of icy worlds beyond Neptune called the Kuiper Belt

- Scientists found Eris... similar to Pluto but even heavier, which forced a big question

- To be a planet, a world must orbit the Sun, be round, and clear its neighbourhood

- Pluto does two out of three... but it hasn't cleared its neighbourhood

- Pluto became a dwarf planet in 2006... and it's still completely incredible

Key Science Ideas:

- Kuiper Belt: A region beyond Neptune full of icy objects

- Planet definition: The three-rule checklist astronomers use to classify worlds

- Clear its neighbourhood: Being the gravitational boss of your part of space

- Dwarf planet: Orbits the Sun, is round, but hasn't cleared its neighbourhood

- Scientific change: Science updates its ideas when new evidence appears

Fun Experiment: The Planet Checklist Challenge

Scientists use a three-rule checklist to decide what counts as a planet. Now it's your turn to be the astronomer.

Grab five objects from around the house... a footy, a mandarin, a marble, a 50 cent coin, and a tin of baked beans. These are your space objects. For each one, ask the three planet questions:

- Does it orbit a star? Walk it around a lamp

- Is it round? Check its shape

- Is it the boss of its space? Could it push everything else out of its way, or does it have to share?

Only objects that pass all three rules get to be a planet. Then here's the twist... change one rule and see what happens. That's exactly what scientists did in 2006. They didn't move Pluto. They just got clearer about the rules. And Pluto couldn't pass the third test.

Why Though? The show for little scientists who love asking big questions. Follow or subscribe so you never miss an episode.

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